1. Field of the Invention
The present invention relates to the development of inexpensive materials and means to apply them, which materials are eminently suitable for the correction of iron deficiency-induced chlorosis in plants. More particularly, the present invention relates to the development of certain materials selected from the group of gel-forming hydrophilic polymers, comprising polyacrylamides, cellulose ethers, guar gums, propenoate-propenamide, or mixtures thereof, said polymer disposed in combination with citric acid, and the resulting combinations in further combination with certain iron source materials including, but not limited to iron sulfates. Still, more particularly, the instant invention relates to the discovery that certain organic acids, particularly citric acid, if added to the aforementioned already hydrated hydrophilic gel-forming polymers or fluid gels of Mortvedt, supra, will significantly increase the efficacy of the hydrated gels in correcting iron chlorosis of plants. Still, even more particularly, the instant invention relates to the discovery that certain formulations of the hydrated gels which are properly combined with predetermined proportions of citric acid, and from which is removed sufficient moisture to result in a friable material from which particulates are easily recovered can, upon subsequent wetting by water in the soil, react to re-form, in situ, such hydrated gel. The resulting re-formed gel complexes and otherwise protects selected iron compounds to provide an economical and readily available iron source imminently suitable for correcting iron deficiencies in plant life growing at such situs. On the other hand, it has been found that the same gel formulations, sans said citric acid, will not perform in such a desirable manner. Such new and improved product is herein designated as "dried iron-containing gel particles;" "dried gel particles;" or more simply "DGP." It has also now been determined that such DGP most preferably should be band applied in a continuous intact band at or prior to planting, or spot placed in the root zone of growing plants in soil to minimize the contact of these products with the soil so that chemical reactions which adversely affect the availability of iron in these products to plants are minimized. When applied in such fashion, the DGP will hydrate and coalesce to form a continuous, intact gel entity, either in the form of a gel band or other such isolated gel area, in essentially the same final form as the gel delivery system of Mortvedt, supra, but with considerably enhanced ease of application. Moreover and most significantly, the instant invention relates to the discovery that the addition of citric acid to such combinations of polyacrylamide polymers and iron-containing materials causes the formation of a compound or compounds, not previously observed to occur with other hydrophilic polymer/iron source materials, which will diffuse out of the hydrated band of DGP into the surrounding soil as evidenced by a diffuse orange-colored zone or "halo", radiating outward from the band into the soil. This zone, as evidenced by the orange coloration, likely contains iron in an oxidized, water-soluble form. Incomplete polymerization of acrylic acid during polyacrylamide synthesis results in free, water-soluble monomers of acrylic acid within the polymer structure. It is speculated that these monomers react with citric acid and iron to form a cyclic, water-soluble iron complex which diffuses from the gel band into the soil. The bonding strength of the complex for iron is sufficient to prevent soil reactions which result in precipitation of iron as compounds which are unavailable to plants. The acrylic acid alone will not form a sufficiently strong complex with iron to prevent these reactions (log K.degree.=4.2 for Fe(II)acrylate). Moreover, and still more significantly, observation of the soil matrix in which this orange-colored zone or halo occurs, clearly shows an unusual propensity of root and root hair growth in the region where DGP was applied, in preference to the surrounding soil matrix. Such root proliferation typically occurs in zones of enhanced fertility in soils, and in such instances where DGP has been applied such enhancement is likely due to a greater concentration of available iron in this zone than in the surrounding soil. This enhanced and concentrated region of root growth occurred to a much lesser extent in like polymer systems which contained iron, but not citric acid, and did not occur at all when iron and citric acid were omitted from otherwise similar formulations.
Furthermore, it has now been discovered that oftentimes the DGP will absorb up to 100 times its own weight in water from contact with moist soil. This process results in swelling of the DGP such that veins or islands of micronutrient-enriched hydrogel are established along with concomitant displacement of the soil around the DGP which results in a zone more easily penetrated and expanded into by plant roots than is a normal soil matrix and which by virtue of copious amounts of water of hydration available to roots growing therein, as well as the abundant supply of micronutrients, provides a region where root growth is substantially enhanced.
In addition, it has now been demonstrated in field testing that such DGP is in a form which may be easily, selectively, and precisely dispensed into soil as a sub-surface band of dry particles by means of a device known in the trade as a pesticide applicator box. This device is designed to contain only small amounts of pesticides, as compared to equipment used for application of major plant nutrients, or "macronutrients", such as nitrogen (N), phosphorus (P), and potassium (K), since such pesticides are normally applied to soil in small amounts (on the order of 1 to 5 pounds per acre, much as with applications of iron) and thus precise metering of the material is required. This precise metering consideration is most important because iron chlorosis resulting from iron deficiencies in soil has been found to occur in separate or isolated areas within any of a number of given fields, which areas range from less than one, up to several acres in dimension, which are oftentimes isolated one from another and which produce little, if any, gainful yield. Unfortunately, currently practiced commercial practices for planting such high pH, calcareous and iron-deficient fields, wherein macronutrients, which are required in large amounts by plants are routinely applied to such areas in a blanket application, do not provide a means which is practical to selectively dispense these micronutrients only in such isolated areas. However, in the practice of the instant invention, if the locations of these iron-deficient areas are known by previous experience, or are otherwise effectively mapped, the DGP can be selectively applied during application of the macronutrients by using a variable rate pesticide applicator box equipped with a banding attachment for subsurface banding of DGP only upon reaching such susceptible areas and not across the whole area of the field. Thus, gainful yields may be realized from areas where before little or none were possible and the added expense of the DGP is therefore more than offset by the increased economic return of such yields.
2. Description of the Prior Art
Iron is an essential element in plant nutrition and generally is classified as a micronutrient. It is known to be involved in the synthesis of chlorophyll which in turn is required for photosynthesis in plants. A deficiency of this micronutrient in growing plants, which can be greatly exaggerated in calcareous type soils, is oftentimes the cause of chlorosis, which is characterized by a yellowing of plant leaves and stems and which results in particularly poor growth.
Currently available practices for alleviating such iron deficiencies in growing plants include the application of synthetic iron chelates to soil or the use of various soluble iron compounds as foliar sprays for direct application to the plants or the use of certain hydrophilic polymer delivery systems. Currently, the least expensive, in terms of up-front per unit cost, water-soluble iron compound in use is iron sulfate, either in its reduced state, e.g., (FeSO.sub.4) or in the ferric state, e.g. [Fe.sub.2 (SO.sub.4).sub.3 ]. However, neither form supra, of iron sulfate should be applied directly to soil lest either source quickly becomes combined with certain components in the soil to form water-insoluble compounds thereby rendering such iron unavailable to growing plants.
The synthetic chelate, FeEDDHA [ferric chelate of ethylenediamine (di-(o-hydroxyphenyl acetate))], has long been considered by many skilled in the art to be the most effective iron fertilizer for soil application, especially in calcareous soils (Arthur Wallace, A Decade of Synthetic Chelating Agents in Inorganic Plant Nutrition, Edwards Brothers, Inc., Ann Arbor, Mich., 1962). However, the per unit cost of iron in FeEDDHA is quite high, which makes this iron chelate material much too expensive for application to relatively low-value field crops. Another currently available and somewhat less expensive iron chelate material, FeEDTA (monosodium ferric ethylenediamine tetraacetate), has proven to be effective for crops growing in near neutral soils but not in calcareous, high-pH soils wherein most iron deficiencies occur. Another recent discovery also somewhat less costly than chelates, are hydrophilic polymer delivery systems (Mortvedt, supra). However, application of the materials of Mortvedt, is difficult and requires specialized equipment and the polymer in the formulations is also relatively expensive. Nevertheless, the initial per unit cost of iron is significantly lower than in the chelates. Accordingly, it should be readily apparent that iron sulfate would be the most economical and eminently suitable iron source material for use on field crops if it only remained available to growing plants subsequent to its contact or juxtapositioning with the soil situs. Therefore, additives or conditions which can significantly improve the effectiveness of iron sulfate intended for the treatment of chlorosis could, in turn, result in an economically effective iron source for soil application.
Currently, it is the practice in the trade for iron source material to be applied to soil separately or to be incorporated with other materials in the processing or blending of fertilizers or to be applied in a hydrophilic gel polymer matrix. The effectiveness of iron source materials in maintaining a supply of iron to growing plants depends upon the chemical nature of such iron source materials and/or the soil, as well as rate and/or frequency of their application. Economic considerations regarding the use of iron source materials are determined by costs and rate of their application as well as the ease of application relative to the returns attributable to increased yields of the crops to which they are applied. Presently, the most effective iron chelate, FeEDDHA, is so costly that its use is restricted to high-cash value crops such as, for example, apples, grapes, and peaches, or high-cash value ornamental crops such as, for example, rhododendrons, azaleas, and dwarf citrus, while other methods, i.e., fluid hydrophilic polymer delivery systems, are nonetheless still expensive in addition to being difficult to apply and are not as effective as FeEDDHA. The least costly, on a front-end per unit cost basis, iron source materials are ineffective when used in procedures designed to correct iron chlorosis in many lower value field crops, such as, for example, corn, grain sorghum and soybean, which nonetheless are planted in large acreage and constitute the major portion of modern production agriculture.
From the aforesaid, it should now be abundantly clear that the prior art materials designed as, or intended to be, iron sources are too costly up front to be economical for use on most field crops or are difficult to apply and require specialized application equipment, or although available at relatively low unit cost, are still highly uneconomical to use since they are ineffective in maintaining a supply of available iron to crops growing on iron-deficient soils.